1. Field of the Invention
This invention relates to a signal recording apparatus and more particularly to an apparatus for recording a video signal and other signals related to the video signal.
2. Description of the Related Art
Research for apparatuses of the kind handling high-definition (hereinafter referred to as high-vision) TV signals have progressed during recent years. As a result, various technical methods have been developed for image sensing, signal transmission, recording and reproduction.
For broadcast transmission, a method called a MUSE (multiple sub-Nyquist-sampling encoding) method has been proposed. A signal to be transmitted by the MUSE method (hereinafter referred to simply as a MUSE signal) is obtained as follows: the band of a high-vision signal is compressed to a great extent. A digital audio signal which has been time-compressed is time-division-multiplexed along with the compressed high-vision signal. The signal thus obtained is converted into an analog signal before it is transmitted through an analog transmission line.
FIG. 1 of the accompanying drawings shows one frame amount of the MUSE signal processed for transmission. As shown, the transmission signal of the MUSE signal consists of 1,125 lines per frame. The signal is transmitted line by line in sequence. In FIG. 1, a reference symbol Y denotes a luminance signal which is obtained by converting a sampled value into an analog signal. A symbol C denotes a chrominance signal. A symbol A denotes an audio signal which is obtained by converting a temporarily digitized audio signal into a three-valued signal. A symbol FP/VIT denotes a signal VIT which indicates information about transmission line equalization, etc., and a frame pulse FP. A symbol GU denotes a guard space. A symbol CON denotes a control signal. A symbol BL denotes blank data. A symbol CP denotes a symbol for clamp level information. Numerical values shown in FIG. 1 indicate the numbers of data obtained in a digital region before the transmission signal is converted into an analog signal.
Further details of each symbol shown in FIG. 1 are disclosed in "High-Vision Technique", Chap. 3, Para. 3.1 and 3.2, issued Nov. 25, 1988, compiled by NHK Broadcasting Technical Research Center. Therefore, the details are omitted from the following description.
In accordance with the MUSE method, the transmission band of the high-vision signal can be lowered a sufficient degree. Besides, the high-vision signal can be transmitted without much degradation of picture quality. The method is advantageous in these respects.
However, a VTR which is arranged to record and reproduce the above-stated MUSE signal on and from a magnetic recording medium presents the following problem.
Generally, a magnetic recording and reproducing system must be arranged taking into consideration the possibility of a drop-out, which results from dust or the like sticking to a recording medium, blinding of a magnetic head, etc. In other words, there is a possibility that the signal becomes unreproducible for a continuous period. In the case of digital recording, for example, this causes a so-called burst error and thus brings about a serious signal deterioration. In the event of such drop-out, the drop-out part of the signal is generally compensated for by an interpolation process utilizing the correlativity of the signal. In the case of a video signal, for example, a signal part adjacent to the drop-out part on the picture is inserted in the drop-out part in such a way as to compensate for the drop-out.
With respect to the audio signal, a short dropout can be compensated for, for example, by holding the level of an immediately preceding part. However, it would be hardly possible to effectively compensate for a drop-out part if it lasts for a long period of time. In that event, the drop-out part is reproduced as a large noise.
In the case of the above-stated MUSE signal, the audio signal is continuously transmitted with each frame-period amount of it time-division-compressed. Therefore, when the MUSE signal is recorded and reproduced, some part of the audio signal might drop out for a considerably long period of time. Under such a condition, the audio signal cannot be adequately reproduced.
This problem is not peculiar to the MUSE signal. It applies particularly to all the apparatuses that are arranged to record a TV signal obtained by time-division-multiplexing a given period amount of video signal with the same period amount of audio signal.
Further, the MUSE signal has a considerably wide band. In transmitting it in a digitized state, therefore, the amount of data thereof is preferably compressed by means of a so-called high-efficiency encoding circuit. However, the MUSE signal consists of a luminance signal, a chrominance signal and the audio signal which are multiplexed in a time sharing manner. Therefore, transmission of them in the high-efficiency-coded state necessitates use of a large circuit arrangement. In other words, for high-efficiency-encoding a signal in which a video signal and an audio signal are multiplexed in a time sharing manner, these component signals must be first separated from each other in general. After that, the video and audio signals thus separated must be processed by separate signal processing systems and then further separately subjected to the high-efficiency encoding process. The signal thus requires a complex processing arrangement.
Further, the video and audio signals included in the MUSE signal are in utterly different signal forms. The signal processing circuits for them must be arranged in totally different manners. In other words, the luminance and chrominance signals are analog signals corresponding to the amplitudes indicated by the respective symbols of the digital signals. Meanwhile, the audio signal is an analog signal obtained by converting a digitized two-valued audio signal into a three-valued signal. Therefore, the digital process circuit elements must be separately arranged to handle a different number of data bits in different manners.
In cases where the MUSE signal is to be transmitted, recorded or reproduced by using an apparatus in common with some other video signal which is a baseband signal or the like obtained by digitizing a high-vision signal as it is, both the video signal and audio signal processing systems of the apparatus are not usable for the other video signal. Therefore, in such a case, the size of the apparatus greatly increases.
Further, generally, digital signals of different bit rates obtained by sampling information signals at different sampling frequencies are recorded by different apparatuses. Even in a case where the difference in bit rate between these different digital signals is not much thus permitting common use of mechanical parts, it has been necessary to separately arrange the signal processing parts for them.
For example, a digital signal which is obtained by band-compressing the baseband signal of a high-vision (high definition TV) signal for recording differs, both in bit rate and signal form, from the digital signal before transmission of a MUSE signal which is prepared for high-vision (high-definition) transmission. Hence, utterly different digital VTRs have been used for recording these digital signals. However, the existence of VTRs of such different kinds is undesirable in terms of the convenience of the users and cost of development.
In view of this, U.S. patent application Ser. No. 228,594 filed on Aug. 5, 1988, and which issued on Apr. 30, 1991 as U.S. Pat. No. 5,012,352 has proposed an apparatus which is arranged to be capable of recording digital signals of different bit rates by making the number of video data recorded in each of recording tracks variable. In the case of this apparatus, the digital signal obtained by band-compressing the high-vision (high-definition) baseband signal and the digital signal of the MUSE signal before transmission can be processed by using the same mechanism and signal processing part.
The above-stated recording apparatus which is capable of recording digital signals of different bit rates by using one and the same apparatus is highly advantageous particularly in respect of its high universal applicability to digital signals of different kinds. However, in a case where a plurality of specific digital signals of different kinds are to be recorded, it still presents some problem that should be solved for facilitation of a data handling process. The following describes this problem by way of example through a digital VTR which is capable of recording digital video signals of two different kinds.
Generally, a digital VTR is arranged to divide video and audio signals into a given number of symbol units and to form a synchronizing block by adding to them an error correction code for correction of any code error occurring in a magnetic recording or reproducing system, identification data (hereinafter referred to as ID data) of varied kinds and synchronizing (hereinafter referred to as sync) data which consists of a given number of symbols provided for recomposing data at the time of reproduction. The video and audio signals are thus recorded on a magnetic tape with the sync block used as the unit of recording.
The sync block is generally arranged, for the purpose of facilitating data handling by the reproducing system, to include a number of symbols of video data in an integral ratio to a number of symbols corresponding to one scanning line amount of data of the video signal.
However, the bit rates of digital video signals of different kinds are seldom in an integral ratio to each other. Any attempt to have them in an integral ratio would impose some restriction on the form of digital signals. Such an attempt is, therefore, undesirable.
In a case where the digital video signals of two different kinds which have their bit rates not in an integral ratio but are to be recorded, the number of symbols of the sync block of one video signal which has a lower bit rate is adjusted to that of the other video signal which has a higher bit rate. In this instance, if the number of symbols of the sync block is set at a value corresponding to the number of symbols of one scanning line amount of the video signal having the higher bit rate, the start position of each line of the video signal having the lower bit rate becomes random in recording the signal. Such recording makes it difficult to recompose the video signal in the event of a special reproduction mode such as a high-speed search or the like.
Further, in cases where a digital signal obtained by band-compressing a high-vision baseband signal (hereinafter referred to as a baseband signal) and another digital signal obtained by digitizing a MUSE signal (hereinafter referred to as a digital MUSE signal) are to be recorded, the digital MUSE signal might be the video signal having a lower bit rate. In that event, there arise various problems.
More specifically, with the MUSE transmission signal which is as shown in FIG. 1 digitized and recorded as the video signal having a lower bit rate by a digital VTR which is capable of recording the baseband signal, the leading symbols of luminance and chrominance signals of each line would be allocated in a random state within the sync block recorded. Then, it is extremely troublesome to recompose by a reproducing system a line having the luminance and chrominance signals coexisting there. Further, the audio signal coexists with the chrominance signal in some line. Therefore, recomposition of the audio signal also requires an extremely complex process.